1. Field of the Invention
The present invention relates to a microstructure and its fabrication method on the field of micromachines. More particularly, the present invention relates to a micro dynamic-value sensor, microactuator and micro optical deflector each having a member torsion-vibrating about a torsion axis.
2. Related Background Art
In recent years, various units have been improved for high function and small size because of development of microelectronics as represented by high integration degree of semiconductor devices. The same is said for an apparatus using a micromachine device (such as a micro optical deflector, micro dynamic-value sensor or microactuator having a member torsion-vibrating about a torsion axis). For example, an image display apparatus such as a laser-beam printer or head-mount display which performs optical scanning by using an optical deflector, and a light-capturing apparatus of an input device such as a bar code reader have been also improved for high function and small size and moreover, application of them to a portable product is desired. Furthermore, not only the application of a micromachine device to the portable product but also improvement of performances of the device such as stability of torsional vibration such as external vibration to noises, impact resistance and service life have been particularly requested to the device in addition to further down sizing of the device for practical use.
For example, Japanese Patent Application Laid-Open No. 09-230275, 10th International Conference on Solid-State Sensors and Actuators (Transducers '99) pp. 1002–1005 is disclosed as a proposal for the above request.
(First Conventional Example)
FIG. 16 is a perspective view showing a micro optical deflector of the first conventional example disclosed in U.S. Pat. No. 5,982,521.
A torsion spring 1005 is set to a housing 1001 by a fixing jig 1002 while it is pulled at a tension. Moreover, a magnet-provided mirror 1003 is fixed nearby the center of the torsion spring 1005 by an adhesive (not shown). The magnet-provided mirror 1003 is made of Ni—Co (nickel-cobalt) or Sm—Co (samarium-cobalt) having a thickness of 0.3 mm, a length of 3 mm and a width of 6 mm. The torsion spring 1005 is made of a superelastic alloy (e.g. Ni—Ti alloy) and has a central portion of about 140 μm in line diameter and about 10 mm in length. Moreover, the portion where the torsion spring 1005 is fixed to the housing 1001 is thicker than the central portion to which the magnet-provided mirror 1003 is fixed, as a result of electroless plating or the like. The fixed portion with the housing serves as a housing fixed portion 1013.
Moreover, a coil 1007 is wound on a core 1006 by about 300 turns. The coil 1007 is fixed to the housing 1001 by a screw (not shown) through a tapped hole 1008 formed on the core 1006 and a hole 1004 formed on the housing 1001. Furthermore, a pulse-current generator 1009 is connected to the both ends of the wound wire of the coil 1007. By supplying a current at, for example 3 V and about 100 mA to the coil, an alternate magnetic field is generated and the magnet-provided mirror 1003 vibrates. A laser beam 1010 emitted from a light source 1011 is reflected from the magnet-provided mirror 1003 and the magnet-provided mirror 1003 resonates and thereby, the laser beam is canned on a plane 1012 to be scanned.
The housing fixed portion 1013 is tapered by coating processing such as electroless plating. Therefore, it is possible to moderate concentration of stress on the housing fixed portion 1013 at the time of driving and moreover, the torsion spring 1005 is prevented from disconnection.
(Second Conventional Example
FIG. 14 is a top view of the hard-disk-head gimbals of the second conventional example disclosed in 10th International Conference on Solid-State Sensors and Actuators (Transducers '99) pp. 1002–1005. The gimbals are set to the front end of a hard-disk-head suspension to elastically allow a magnetic head to roll and pitch. The gimbals 2020 have a support frame 2031 rotatably supported by roll torsion bars 2022 and 2024 inside. Moreover, a head support 2030 rotatably supported by pitch torsion bars 2026 an 2028 is formed inside the support frame 2031. Torsional axes (refer to the orthogonal chain lines in FIG. 14) of the roll torsion bars 2022 and 2024 and pitch torsion bars 2026 and 2028 are orthogonal to each other and take charge of roll and pitch of the head support 2030 respectively.
FIG. 15 is a sectional view taken along the cutting-plane line 2006 in FIG. 14. As shown in FIG. 15, the sectional shape of the torsion bar 2022 is T-shaped and the gimbals 2020 are constituted so as to have a rib.
As shown in FIG. 15, the torsion bar having the T-shaped cross section has a large moment of inertia of the cross section though it has a small polar moment of inertia of the cross section compared to the case of a torsion bar having a circular cross section or rectangular cross section. Therefore, it is possible to provide a torsion bar which is not easily deflected though comparatively easily twisted. That is, it is possible to provide a torsion bar having a high stiffness in the direction vertical to the torsion axis while securing a sufficient compliance in the torsional direction.
Moreover, there is an advantage that it is possible to further downsize a torsion bar because it is possible to provide a short torsion bar for obtaining a necessary compliance.
Thus, by using the above torsion bar having a T-shaped cross section, it is possible to provide microgimbals which have a sufficient compliance in roll and pitch directions and a sufficient stiffness in other directions and which can be further downsized.
However, the first and second conventional examples have the problems described below.
In the case of the first conventional example, the torsion spring 1005 is a wire rod and its sectional shape is circular. A microstructure having a torsion spring of the above sectional shape has a problem that the structure cannot be accurately driven because its torsion spring is easily deflected to receive the structure external vibrations or move the torsion axis of the torsion spring.
Moreover, because the torsion spring 1005 is easily deflected due to an external impact, there is a problem that the magnet-provided mirror 1003 is greatly displaced in the translational direction (that is, direction vertical to torsion axis) and thereby, a trouble that the torsion spring 1005 is broken easily occurs.
Therefore, when applying the above micro optical deflector to a light scanning display, there is a problem that an image is deformed due to external vibrations or a spot shape is changed. Moreover, there is a problem that a display is broken due to an impact. This leads to a larger problem when a light scanning display is formed into a portable type.
Moreover, in the case of the first conventional example, the torsion spring 1005 is formed so that the wire diameter of the housing fixed portion 1013 fixed to the housing 1001 becomes large for the support portion which supports the magnet-provided mirror 1003. Stress concentration caused by torsional vibration also occurs in the housing fixed portion 1013. However, because the torsional vibration is a relative movement of the magnet-provided mirror 1003 to the housing 1001, stress concentration also occurs in the support portion which supports the magnet-provided mirror 1003 of the torsion spring 1005. Therefore, the first conventional example has a problem that stress concentration on the support portion supporting the magnet-provided mirror 1003 in the torsion spring 1005 cannot be moderated and thereby, the effect of preventing disconnection of the torsion spring 1005 cannot be sufficiently expected.
Finally, the sectional shape of the portion of the torsion spring 1005 to be mainly displaced in the torsional direction is circular and the housing fixed portion 1013 is designed so as to obtain the effect of preventing disconnection by further increasing the wire diameter from the above portion to be mainly displaced. However, there is a problem that the housing 1001 for fixing the housing fixed portion 1013 must be also increased in size because of the structure of the housing fixed portion 1013. Particularly, to downsize a micro optical deflector, dimensions including the thickness of the housing 1001 and the wire diameter of the torsion spring 1005 become larger problems because they become similar on order.
The second conventional example has a problem that the T-shaped-cross-sectional torsion bar is easily broken because stress is concentrated on the support portions at the both ends of the torsion bar (for example, the support portion for the head support 2030 and the support portion for the support frame 2031 in the roll torsion bars 2028 and 2026, or the support portion for the support frame 2031 and the support portion for the gimbals 2020 in the roll torsion bars 2022 and 2024). Therefore, unless the torsion bar is set long enough, it is impossible to drive the torsion bar at a large displacement angle. Thereby, not only downsizing is impossible but also the torsion bar is easily deflected even if greatly lengthening the torsion bar and the head support 2030 is greatly translated in the direction vertical to the torsion axis due to an external impact. Therefore, when mounting the hard-disk-head gimbals of the second conventional example on a hard disk, trouble occurs in the hard disk because the gimbals contact with a recording medium due to an external vibration or impact or a head is broken. This becomes a larger problem when the hard disk is formed into a portable type.
Moreover, there is a problem that a large stress is repeatedly loaded due to the above stress concentration even if a breakage does not occur and thereby, a torsion bar easily early causes a fatigue failure due to a repetitive stress.
The present invention has been accomplished to solve the above conventional problems and its object is to provide a compact microstructure having less unnecessary vibrations and a long service life even at a large torsional angle and its fabrication method and an optical apparatus using the microstructure.